The present invention relates to a method for improved detection and correction of errors in pitch period estimates of speech signals.
In electronic processing of speech signals, for example, in mobile radio, maritime, aircraft and satellite communications speech coders are often employed. Examples of such speech coders include parametric and hybrid speech coders such as Linear Predictive Coders and Adaptive Predictive Encoders.
An example of a Linear Predictive Coder (LPC) is shown in the block diagram of FIG. 1. Incoming 12-bit speech samples are applied to an LPC analysis circuit 1 for vocal cavity modeling, to a voice and pitch analysis circuit 3, and to an energy matching circuit 4. The LPC analysis circuit 1 outputs LPC parameters a.sub.1, . . . a.sub.p, to a quantizer and error control circuit 2, other inputs to which include signals from the voicing and pitch analysis circuit 3 indicative of whether the speech is voiced or unvoiced and its pitch period when voiced, and a gain parameter from the energy matching circuit. The present invention is employed in the voicing and pitch analysis circuit 3. Since, however, the overall system depicted in FIG. 1 is not the direct subject of the present invention and examples of such circuits are well known in the prior art, its details will not be discussed further here.
In these coders, it is usually necessary for the voicing and pitch analysis circuit 3 to provide estimates of the speech pitch period of the speaker and to detect and correct errors in the estimates. The invention relates directly to a method for detecting and correcting in the errors in the pitch period estimates. The pitch period estimates themselves are derived with a device and method distinct from that of the present invention.
Pitch period estimates of speech signals are susceptible to two types of error--gross pitch errors and fine pitch errors. Gross pitch errors, which are large in magnitude, typically arise due to pitch period doubling or background noise. Gross errors are perceived as distorted speech spurts that are subjectively very objectionable. On the other hand, fine pitch errors, which are much smaller in magnitude, are generally caused by limited resolution of the pitch estimation technique or time variations in the pitch period. Fine pitch errors are more tolerable, but result in the perception of a reduced natural quality to the speech. The present invention is concerned primarily with detection and correction of gross errors.
Previous methods for detecting and correcting gross errors in pitch period estimates operated primarily using median smoothing. That is, each pitch period estimate is replaced by a weighted average of itself and its neighboring estimates. All estimates are subjected to smoothing in this manner. In a somewhat more sophisticated scheme, smoothing is performed selectively. Specifically, only if an estimate differs from the average of its neighbors by more than a predetermined amount is the estimate replaced by its smoothed value.
In the first method, the gross errors are reduced at the expense of reducing the accuracy of all estimates, as a result of which fine pitch errors are introduced in all estimates. In the second method. uncorrected gross errors can cause further gross errors.
It is thus an object of the present invention to provide a method for detecting and correcting errors in speech pitch estimates which provides an improved accuracy to the estimates, and which consequently results in the elimination of the difficulties mentioned above.